1. Field of the Invention
This invention relates to measuring optical fibers, and more particularly, to determining the ratio of core radius to cladding radius in a clad optical fiber.
2. Description of the Prior Art
It is known to measure various parameters of an optical fiber by directing a coherent light beam such as a laser beam at the fiber and analyzing the far-field forward scattering pattern thus produced. The outer radius of such a fiber can be determined by counting the number of fringes in a particular region of the scattering pattern. For a clad fiber, the ratio of core radius to cladding radius can be determined by measuring the position of an angle in the scattering pattern where modulation of the fringes starts, thus enabling the core radius to be found from the outer cladding radius. These methods depend on knowing the refractive index of an unclad fiber, or of the cladding of a clad fiber. Such measuring methods are disclosed in a copending application of L.S. Watkins, Ser. No. 482,707, filed June 21, 1974, issued as U.S. Pat. No. 3,982,816 on Sept. 28, 1976, which is assigned to this assignee. Methods and apparatus useful for detecting fringes in a scattering pattern are disclosed in copending application Ser. No. 634,717, filed Nov. 24, 1975 of L.S. Watkins, which is also assigned to this assignee.
Given that the ratio of core radius to cladding radius can be determined from the angle in the forward scattering pattern at which fringe modulation begins, it is desirable to provide a method and apparatus for automatically determining this angle.